1. Field of the Invention
This invention relates to an image sensing device which is advantageously adapted for a video camera, electronic still camera or the like.
2. Description of the Related Art
In the following description, an automatic focusing device is taken up by way of example as the image sensing device. The automatic focusing devices of cameras have been arranged to operate in varied manners. In the case of an apparatus of the kind having image sensing means for obtaining a video signal by photo-electrically converting the image of a picture-taking object, such as a video camera or an electronic still camera, the automatic focusing device is arranged to detect the definition of the object image through the video signal and to make focus adjustment to maximize the degree of definition of the image.
Generally, the apparatus of the above-stated kind is arranged to set a focus detecting area in a part of an image sensing plane. A focus detecting action is performed on an image formed within this area. During recent years, apparatuses of the kind having an automatic object tracing function have been proposed. The apparatus is arranged to be capable of automatically shifting the focus detecting area while tracing the movement of an object for increased performance and functions of the apparatus. This apparatus has been disclosed, for example, in Japanese Laid-Open Patent Application No. SHO 60-249477.
Various methods also have been proposed for tracing the movement of the object. In one of them, for example, the moving position of the object is detected by detecting, as a featuring point of the object image, the peak value of a high-frequency component of the video signal or the like obtained from within the focus detecting area for every field or frame; and, after that, the focus detecting area is newly set in a position where the moving position of the object is located approximately in the center of the area. This method enables a focusing action to be continuously performed on a moving object.
In the apparatus of the above-stated kind, the focus detecting area is arranged to be unvarying in size and to move at an unvarying speed in tracing the object for every field irrespective of picture taking (shooting) conditions. Assuming that the object tracing action is performed by detecting the peak value of the high-frequency component as a feature point of the object, it is difficult to distinguish a main object from a background object in the event of a deep depth of field. A faulty action tends to be caused, under such a condition, by a perspective confusion or an ambiguous relation between the focus detecting area and the object.
Further, in the case of the apparatus of the above-stated kind, the focus detecting area continuously moves in pursuit of the object irrespectively of the shooting conditions. Therefore, in cases where the main object goes out of the image sensing plane because of the movement of the object or the camera itself and where an excessively blurred state is caused by a faulty action of the automatic focusing device, the object having the feature point no longer exists within the focus detecting area. In such a case, the movement of the focus detecting area might be too unstable for continuously performing an accurate focusing action on the object.
Previous patents and patent applications related to the above-stated object tracing devices of the prior art include: U.S. Pat. No. 4,872,058 and U.S. patent application Ser. No. 737,163 filed on May 23, 1985; Ser. No. 154,078 filed on Feb. 9, 1988; Ser. No. 237,511 filed on Aug. 26, 1988; and Ser. No. 240,915 filed on Sep. 6, 1988.